Borehole stress-property measuring system

ABSTRACT

A stress-property measuring system for in situ determination of stress states and material properties of soil and rock media into which the apparatus is disposed via a borehole. The apparatus used in carrying out the instant method comprises two axiallyaligned cylindrical chambers. The first of said chambers is capable of exerting fluid pressure in a transaxial direction relative to the chamber and normal to the inner cylindrical surface of said borehole; the second of said chambers is completely unpressurized and pressure-insulated from said first chamber. Both chambers are fitted with a plurality of sets of diameter-measuring transducers for measuring changes in borehole diameter at a number of points in one diametrical direction along the borehole longitudinal cylindrical axis. Using this measuring device, stress state and material properties of surrounding ground medium may be determined as follows. Fluid pressure is introduced into said first pressurized chamber, and is varied through a gradient of ever-increasing pressures up to a predetermined pressure limit. These ever increasing pressurizations cause corresponding incremental changes in borehole diameters in various radial directions in both the pressurized and non-pressurized sections of said borehole, the magnitude of such changes being determined by the stress conditions and material properties of the surrounding medium. By measuring the diameter change indicated by said diametermeasuring transducers, sufficient and accurate data may be collected to enable computation of the various in situ stress states and material properties of the ground medium.

United States Patent 91 Serata Mar. 12, 1974 BOREHOLE STRESS-PROPERTYMEASURING SYSTEM [76] Inventor: Shosei Serata, 14 Calvin Ct.,

Orinda, Calif. 94563 [22] Filed: Oct. 2, 1972 [21] Appl. No.: 294,431

[52] US. Cl 73/88 E [51] G01b 7/24 [58] Field of Search 73/88 E, 84;33/178 F Primary Examiner-Charles A. Ruehl Attorney, Agent, orFirm-Harris Zimmerman [57] ABSTRACT A stress-property measuring systemfor in situ determination of stress states and material properties ofsoil and rock media into which the apparatus is disposed via a borehole.The apparatus used in carrying out the instant method comprises twoaxially-aligned cylindri- Germany 73/88 E cal chambers. The first ofsaid chambers is capable of exerting fluid pressure in a transaxialdirection relative to the chamber and normal to the inner cylindricalsurface of said borehole; the second of said chambers is completelyunpressurized and pressure-insulated from said first chamber. Bothchambers are fitted with a plurality of sets of diameter-measuringtransducers for measuring changes in borehole diameter at a number ofpoints in one diametrical direction along the borehole longitudinalcylindrical axis. Using this measuring device, stress state and materialproperties of surrounding ground medium may be determined as follows.Fluid pressure is introduced into said first pressurized chamber, and isvaried through a gradient of ever-increasing pressures up to apredetermined pressure limit. These ever increasing pressurizationscause corresponding incremental changes in borehole diameters in variousradial directions in both the pressurized and non-pressurized sectionsof said borehole, the magnitude of such changes being determined by thestress conditions and material properties of the surrounding medium. Bymeasuring the diameter change indicated by said diameter-measuringtransducers, sufficient and accurate data may be collected to enablecomputation of the various in situ stress states and material propertiesof the ground medium.

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sum 3 or a FIG 8 t FIG 9 BACKGROUND OF THE INVENTION Accuratemeasurement of the in situ stress states and material properties ofground media is the most fundamental requirement for quantitativeanalysis and sound design of ground structures such as dams,foundations, slopes, tunnels and underground mines. The engineeringdesign of these ground structures is not based on the actual stressstates nor accurately determined material properties of the groundsinvolved because there is currently no apparatus or system available forefficient and accurate determination of the stress states and materialproperties. As a result, engineering designs are usually based on anassumed stress state and approximated material properties, whichassumption and approximation comprise the main inadequacy of currentengineering practice on design and analysis of the ground structures.

Numerous methods have been developed and tried in the past to determinethe in situ stress state in a ground medium, generally consisting offive separate fundamental steps to measure a stress state in a givenpoint in the medium. First, a deformation gage, generally a boreholediameter gage, is placed on or in the ground media for which the stressis to be determined. Second, a change of the ground loading condition isintroduced by means of additional cutting near the ground such asovercoring, advance coring and slot cutting, and the deformation causedby said cutting is measured by said deformation gage. Third, a specimenof the ground medium of which the measurement is being made is taken,usually in the form of core samples. Fourth, laboratory testing of saidcore samples is conducted to determine property coefficients of thematerial under various triaxial loading conditions. This is a laborioustimeconsuming task requiring a number of specimens for a given materialfrom one test site. Fifth, by using the coefficients thus determined ina laboratory, the stress condition is calculated from the deformationcaused by the applied loading.

The above five-step method has been found to be not sufficientlyaccurate for many engineering applications, in spite of the extensivework requirement, because the core samples are often disturbed in theprocess of coring, transporting, storing and the machining requiredprior to the laboratory testing and, therefore, the core sample as itexists in the laboratory is not representative of the medium in itsnatural state underground. In fact, in many natural ground media, suchas soils and fractured rocks, a sampling of an undisturbed specimen isan impossible task. Furthermore, discontinuities and isotropy which areoften encountered in underground media cannot be evaluated effectivelyin the conventional stress measuring method.

It is, therefore, a purpose of this invention to provide a system whichwill accomplish the simultaneous measurement of the in situ stress stateand the in situ material properties in a ground medium in one in situmeasurement in a single borehole without the need for additional fieldcutting and/or laboratory testing, and which is applicable for themeasurement of a wide range of underground media ranging from soft claysto hard rocks.

It is a further purpose of this invention to provide a method andapparatus which can detect in situ the existence of discontinuities andanisotropy in underground media.

THE DRAWINGS FIG. 1 is a front elevational view of the apparatus of thepresent invention as installed in situ in a test borehole;

FIG. 2 is an end view of the apparatus as represented in FIG. 1;

' FIG. 3 is a diagrammatic representation of the borehole deformationand the stress contour envelopes created during the application ofpressure to the apparatus;

FIG. 4 is an enlarged front crosssectional view of a portion of thepressurizable chamber and illustrating one of the transducers;

FIG. 5 is a cross-sectional view taken along 5-5 of FIG. 4;

FIG. 6 is another front cross-sectional view illustrating the partitionseparating the pressurizable chamber from the non-pressurizable chamber;

FIGS. 7 through 9 are diagrammatic representations of boreholedeformations in different axial positions of the apparatus in abore-hole adjacent a discontinuity; and

FIG. 10 is a diagrammatic representation of another embodiment of theinvention.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to FIGS. 1 and 2, themeasuring device of the present invention, generally designated by thenumeral 2, is inserted axially along and into a test borehole 4 cut intothe underground medium 5 to be evaluated. In FIG. 1, the direction ofinsertion is from left to right into the borehole, with anon-pressurizable cylindrical chamber 6 of the device preceding apressurizable cylindrical chamber 8 into the borehole. Both chambers 6and 8 are provided with a plurality of sets of a number of diametermeasuring transducers, which are spaced along the respective cylindricalaxes of chambers 6 and 8, and oriented in a common diameter directionperpendicular thereto, and which are adapted to respond to diameterchanges in the borehole 4 in a transaxial direction within the borehole.Shown in FIG. 1 is a system equipped with two mutually perpendicularsets of the transducers. Although these two sets are sufficient in auniform ground where the principal stress directions are known, ageneral case requires at least three sets in three differentdiameter-axis planes respectively. A system with more than three sets ofthe transducers is needed in grounds where the material properties arenon-uniform and anisotropic. In general, the greater the number of thesets, the greater the accuracy of the measured results. Pressurizablechamber 8, designed to exert controlled variable fluid pressure againstthe borehole wall 14, is mounted in fixed spatial relationship tonon-pressurizable chamber 6 along a common cylindrical axis 16 which isgenerally the central cylindrical axis of the borehole 4.Non-pressurizable chamber 6 is effectively insulated from the fluidpressure of pressurizable chamber 8 by a separating wall arrangement,the detail of which is best shown in FIG. 6. Fluid pressure inpressurizable chamber 8 is controllable by conventional remote reservoirand pump devices (not shown) connected to chamber 8 by a conduit 18.

Details of the mounting of the diameter measuring transducers aregenerally represented in FIGS. 4 and 5 illustrating an embodimentemploying a standard linear variable displacement transformer (LVDT)mounted in the pressurizable chamber 8 to measure borehole diameterchanges. The LVDT 20 is mounted freely slidably in a hole boredtransversely diametrically through a steel cylinder 24 which forms thebasic structural'member of the pressurizable chamber. Two diametricallymovable parts of the LVDT, magnet 21 and solenoid 22, are fastenedseparately to end pads 25 and 26 which are mounted to metal sleeves 27and 28 respectively. The sleeves are bonded to expandable jacket 34 sothat said pads may respectively move, by regulation of the liquidpressure, in directions parallel to the major cylindrical axis of theLVDT, as indicated by arrows a and b. The modulated electrical signaloutput from the LVDT, calibratable to borehole diameter changes, is fedto appropriate remote metering devices by a multiple wire conduit 30which extends from the LVDT through the central primary bore 32 of thepressurizable chamber 8 to the remote station via the fluid conduit 18.

With further reference to FIGS. 4 and 5, the structure of thepressurized chamber 8 is basically as follows. The basic structuralmember is a steel cylindrical tube 24 with an inner primary cavity bore32 which receives controllable fluid pressure via the aforementionedfluid conduit 18. A semi-rigid, expandable rubberized envelope jacket 34is disposed in spaced relation to tube 24 and defines an annular cavity36 therebetween. In operation, the outer surface 38 ofjacket 34 will becontiguously disposed to inner surface 14 of borehole 4. Fluid transferpassages 40 extend radially through inner tube 24 so as to transferfluid pressure from primary cavity 32 to secondary annular cavity 36,thereby applying said pressure to borehole surface 14 via the expandablejacket 34.

Non-pressurizable chamber 6, as generally shown on the right in FIG. 6,includes a rigid cylindrical housing 41 defining an inner chamber 43 forreceiving the necessary electrical leads. A plurality of LVDTs extenddiametrically through the housing in the same manner as such transducersextend through the construction of chamber 8. It will be understood thatthe chamber 43 may be open to the atmosphere or submerged in water orsome borehole fluid but is not intended to receive any internal fluidpressure from the pressurizable chamber. In operation, the patterns ofdiameter changes are obtained in relation to the fluid pressure of thepressurizable chamber as a function of the time of loading and unloadingas follows. The pressure level in the pressurized chamber is graduallyraised by repeating load-unload cycles until the applied pressure levelexceeds the sum of the in situ earth pressure and strength of the groundmedium. The excess pressure thus applied will destroy the borehole.Borehole deformations observed throughout the entireloading process uptofailure point disclose the total spectrum of the ground medium behaviorincluding initial consolidation, elastic and non-elastic deformation,yield, fracture, viscoelastic deformation, plastic flow and finalfailure. During the test process, borehole diameter changes are measuredaccurately across the partition wall (dividing cylinders 6 and 8 fromeach other) at the various locations determined by the LVDTs in thepressurizable and non-pressurizable chambers in relation to the loadingpressure levels. The end result is a set of in situ simultaneous data,including the loading, pressure, deformation with respect to time in thepressurized and non-pressurized sections of the borehole, providinginformation sufficient and necessary for computing the stress statecondition and material structural properties of the ground, such aselasticity, viscoelasticity, viscoplasticity, compressibility, yield andfailure strengths.

Reference to FIG. 3 will assist in conceptual understanding of theprinciple of operation of the system of this invention. Inserted intoborehole 4 are pressurizable chamber 8 and non-pressurizable chamber 6.Chamber 6 is preferably connected to cylinder 8 such as by internalthreads provided at an end of housing 41 engaging external threadsprovided on an extension 45 provided on chamber 8. The expandable jacketof chamber 8 will be in contact with a portion of borehole surface 14.Once chamber 8 is pressurized to a pressure represented by arrows c, thestress develops in the surrounding ground as indicated by stress contourenvelopes, represented by arrows d. Then, the ground moves inperpendicular to the envelopes as indicated by curves e. The groundmovement results in the borehole deformation so that borehole diameterdeformation profiles 42 and 44 develop. These profiles are measured bythe LVDTs positioned along both the pressurized chamber 8 and thenon-pressurized chamber 6 in a number of transaxial directions. It isnoted that the borehole diameter deformation will be positive along theextent of the pressurized region, and will actually become negativeoutside of the region with a relatively large magnitude of thedeformation for an extent of approximately one or two diameters alongthe borehole, beyond which distance the deformation vanishes rapidly.While deformation will be measured generally along the total extent ofboth the pressurizableand non-pressurizable cylinders, it is importantthat the disposition of the LVDTs on the non-pressurizable chamber isstarted immediately next to the separating wall between thepressurizable and non-pressurizable chambers. The distance between theseparating wall and the first LVDT transducer, represented as dimensionL in FIG. 3, must be kept as close to zero as possible so as to permit areading of sharply varying borehole diameter change to be made in thenonpressurized section of the borehole as close as possible to theseparation wall which represents the transition plane from positivepressure to zero pressure exerted in the borehole. The

closer this dimension approaches zero, the more accurate will be thedata obtained from the test, and the more closely will the calculatedmaterial coefficients approach true in situ coefficients.

FIG. 6 illustrates internal detail of the partition wall between thepressurizable and non-pressurizable chambers 8 and 6 which is especiallydesigned for minimizing the L value. Forming the principal structuralmember of the pressurizable section 8 is the cylindrical tube 46preferably constructed of high strength steel to withstand fluidpressure of up to 50,000 psi which will be required to perform analysisof hardest rock media. At one axial extremity of said tube 46 is formeda flangelike projection 48 which constitutes the partition wall per sebetween the cylinders. Machined axially into said projection 48 are aplurality of conductor grooves 50 through which are passed individualoutput wire leads 52 of the individual LVDTs positioned in thenon-pressurizable section 6. In this preferred'embodiment of the instantinvention, pressure leakage through the conductor grooves 50 isprevented by encapsulating wire leads 52 with a bi-compositional plugcomposed of a hard epoxy cement element 54 and a soft epoxy cementelement 56. Enveloping the pressurizable cylinder 8 is the rubber orplastic composition jacket 34 into which is molded a cylindrical brushof fine, free-ended parallel piano wires 58 crimped upwardly, inwardly,and backwardly at the two lateral ends of the pressurized chamber. Thiscomposite rubber/wire brush arrangement serves the following functionsin the instant device. First, it secures the cylinder 8 in position bythe friction developed between the expandable jacket and the boreholesurface. Second, it pressure-seals the space between the outsidediameter of the expandable jacket and the borehole wall through theflexibility the resiliency of the composite rubber/- wire jacket. (Notethat an additional sealing wedge, not shown, could be employed to fit alarger borehole.) Third, it insulates the diameter-measuring transducersfrom internal axial tension, as the wires themselves, rather than therubber which surrounds the transducers, will carry most of the shearingstresses. Fourth, it reinforces the rubber jacket against cracking,splitting, etc.

Another embodiment of the system of this invention, diagrammaticallyillustrated in FIG. 10, is that of employing two sets of pressurizableand non-pressurizable cylinders in series in the borehole. Such anarrangement will permit more accurate measurements to be taken becausethe magnitude of all borehole diameter deformations will becorrespondingly increased, thereby improving transducer performance andresponse as well as increasing the total volume of surrounding groundmedia involved in a single testing.

It is also noted that the system of the instant invention is capable ofanalyzing both anisotropy and discontinuity of in situ stress state andmaterial properties as diagrammatically illustrated in FIGS. 7, 8 and 9.These figures illustrate diameter change distribution curves plottedvertically against borehole depth. Line M in each figure represents adiscontinuity plane between relatively soft ground on the left andrelatively hard ground on the right. In FIG. 7, the device of thisinvention lies entirely to the left of the discontinuity. In FIG. 8 thedevice straddles the discontinuity. And, in FIG. 9, the device liesentirely to the right of the discontinuity. By incrementally advancingthe device into borehole sections of suspected discontinuity, theexistence or non-existence of the discontinuity, as well as itslocation, can be ascertained merely by inspection of the resultant datafrom the LVDTs. Further, the bonding properties of the discontinuitycontact surfaces can be accurately calculated from the diameterdeformation data in the same manner described above for determining insitu material properties of continuous media.

I claim:

1. A method of analyzing stress states and material properties of groundmedia surrounding a borehole which consists of applying radially outwardpressure to said borehole only along a first portion of the lengththereof, positioning a plurality of transducers in said boreholetransaxially of the length thereof in said first portion of the boreholeand in a second portion of the length of said borehole adjacent saidfirst portion, and measuring the radial deformation of said borehole insaid first and second portions.

2. The method of claim 1 in which said transducers lie in a plurality ofdiameter-axis planes in each of which a set of a number of saidtransducers are disposed to observe the distribution curve of thediameter change along the borehole axis.

3. The method of claim 1 in which said pressure is incrementallyincreased, and said measurements are taken during successive incrementalincreases in pressure with respect to time of the pressurizing.

4. The method as set forth in claim 1 in whlch said pressure is appliedto said borehole substantially uniformly along said first portion whilenot permitting any such pressure to be applied along said secondportion.

5. The method Of claim 4 in which said transducers are closely disposedin said borehole in said second portion starting immediately next tosaid first portion at an axial distance from said first portion nogreater than twice the diameter of said borehole, so that the mostcritically changing portion of the diameter change distribution curvelies within the distance from the juncture of said portion twice thediameter of said borehole as observed in the individual diameter-axisplanes.

6. Apparatus for analyzing stress states and material properties ofground media surrounding a borehole, comprising a first tubular memberand an axially aligned second tubular member, said first member having aradially expandable outer portion, means for selectively expanding saidportion, a plurality of transaxially disposed length-measuring devicescarried by said first and second members and normally extending beyondthe outer surfaces thereof, each said lengthmeasuring device measuringthe diameter of said borehole at the location of each said device.

7. Apparatus as set forthin claim 6 in which said length-measuringdevices comprise transducers spaced axially of each of said members.

8. Apparatus as set forth in claim 7 in which said transducers aredisposed in a common transaxial direction in a plurality ofdiameter-axis planes.

9. Apparatus as set forth in claim 7 in which said transducers haveelectrical leads extending through said first member, said first memberhaving a pressure chamber therein, and means sealing said first memberfrom said second member.

10. Apparatus as set forth in claim 7 in which said transducers compriselinear variable displacement transformers.

11. Apparatus as set forth in claim 6 in which said first and secondmembers are secured together, and said first member has an inner fluidpressure chamber therein for expanding said expandable outer portion.

12. Apparatus as set forth in claim 11 in which said first chamber issubstantially sealed by a partition wall which enables fluid pressure tobe exerted on the surface of said borehole up to the extent of the firstcham her and, at the same time, enables the said lengthmeasuring devicesof the second chamber to be disposed substantially immediately next tothe limit of pressurization.

1. A method of analyzing stress states and material properties of groundmedia surrounding a borehole which consists of applying radially outwardpressure to said borehole only along a first portion of the lengththereof, positioning a plurality of transducers in said boreholetransaxially of the length thereof in said first portion of the boreholeand in a second portion of the length of said borehole adjacent saidfirst portion, and measuring the radial deformation of said borehole insaid first and second portions.
 2. The method of claim 1 in which saidtransducers lie in a plurality of diameter-axis planes in each of whicha set of a number of said transducers are disposed to observe thedistribution curve of the diameter change along the borehole axis. 3.The method of claim 1 in which said pressure is incrementally increased,and said measurements are taken during successive incremental increasesin pressure with respect to time of the pressurizing.
 4. The method asset forth in claim 1 in whIch said pressure is applied to said boreholesubstantially uniformly along said first portion while not permittingany such pressure to be applied along said second portion.
 5. The methodOf claim 4 in which said transducers are closely disposed in saidborehole in said second portion starting immediately next to said firstportion at an axial distance from said first portion no greater tHantwice the diameter of said borehole, so that the most criticallychanging portion of the diameter change distribution curve lies withinthe distance from the juncture of said portion twice the diameter ofsaid borehole as observed in the individual diameter-axis planes. 6.Apparatus for analyzing stress states and material properties of groundmedia surrounding a borehole, comprising a first tubular member and anaxially aligned second tubular member, said first member having aradially expandable outer portion, means for selectively expanding saidportion, a plurality of transaxially disposed length-measuring devicescarried by said first and second members and normally extending beyondthe outer surfaces thereof, each said length-measuring device measuringthe diameter of said borehole at the location of each said device. 7.Apparatus as set forth in claim 6 in which said length-measuring devicescomprise transducers spaced axially of each of said members. 8.Apparatus as set forth in claim 7 in which said transducers are disposedin a common transaxial direction in a plurality of diameter-axis planes.9. Apparatus as set forth in claim 7 in which said transducers haveelectrical leads extending through said first member, said first memberhaving a pressure chamber therein, and means sealing said first memberfrom said second member.
 10. Apparatus as set forth in claim 7 in whichsaid transducers comprise linear variable displacement transformers. 11.Apparatus as set forth in claim 6 in which said first and second membersare secured together, and said first member has an inner fluid pressurechamber therein for expanding said expandable outer portion. 12.Apparatus as set forth in claim 11 in which said first chamber issubstantially sealed by a partition wall which enables fluid pressure tobe exerted on the surface of said borehole up to the extent of the firstchamber and, at the same time, enables the said length-measuring devicesof the second chamber to be disposed substantially immediately next tothe limit of pressurization.